1. Field of the Invention
The present invention relates generally to the simulation of electronic circuits and, more particularly, to the high frequency modeling of MOSFET circuit elements.
2. Background Information
MOSFET technology was originally developed for use in DC and low frequency applications. As early versions of this technology were incapable of operating properly at higher frequencies, bipolar junction and GaAs transistors were used for radio frequency applications. However, as MOSFET process technology has moved into the deep submicron region, the cut-off frequencies of such devices has increased into the tens of gigahertz, making MOSFET technology a serious alternative for high frequency circuit integration.
The use of entirely MOSFET technology for implementing circuits for radio frequency applications allows for maximum integration of the RF front end, baseband logic, custom analog, and memory modules for complete systems on a single chip. Integration not only reduces device size, but allows system manufacturers to drive manufacturing costs down.
An important tool in the design of such large integrated circuits are methods of circuit simulation, the most familiar being SPICE. To run a SPICE or other circuit simulation, the circuit designer provides a description of the circuit, choosing a model for the various elements and specifying the parameter values, and the desired analysis, which specifies what sort of simulation will be performed in order to provide the desired output. This information forms a netlist which the designer runs to analyze the circuit.
The simulation tools available for non-linear MOSFET devices reflect the origin of this technology in DC and low frequency applications. While MOSFET devices now possess the performance needed for high frequency operation, the available design tools have yet to fully learn and embrace the intricate physical phenomena of such high speeds of operation. Without access to such “RF-ready” design tools, designers are hard pressed to design products that meet the tight constraints on power consumption and noise that leave very little margin for error.
One technique used in simulation is to replace non-linear elements in the netlist with a sub-circuit. Although it is possible to use a detailed equivalent circuit that accounts for all the physical elements that are part of a MOSFET transistor operating at high frequencies, the result is generally too complex to implemented as a compact model or sub-circuit for simulation. Additionally, many of the component values would be difficult or even impossible to extract and the resultant sub-circuit would contain a large number of internal nodes, greatly increasing the simulation time. Current techniques for the production of a sub-circuit for use in simulation of MOSFET circuits are extremely slow and often provide inaccurate results when pushed into the RF region. What is required is a MOSFET model that can accurately extend well into the gigahertz range, be quick, and still give accurate DC and low frequency AC fitting.
Another important consideration in circuit design is noise. In addition to providing a unified design tool that can accurately describe MOSFET operation in the DC region as well as its high frequency behavior, the model should preferably incorporated noise considerations. In this way, the designer can simultaneously consider all of these effects and emphasize those most important to overall circuit's characterization.